This invention relates to a floating gate circuit, and more specifically to a method and apparatus for sensing the analog voltage on a floating gate while the floating gate is being set to a desired voltage, so as to precisely control said voltage.
Programmable analog floating gate circuits have been used since the early 1980""s in applications that only require moderate absolute voltage accuracy over time, e.g., an absolute voltage accuracy of 100-200 mV over time. Such devices are conventionally used to provide long-term non-volatile storage of charge on a floating gate. A floating gate is an island of conductive material that is electrically isolated from a substrate but capacitively coupled to the substrate or to other conductive layers. Typically, a floating gate forms the gate of an MOS transistor that is used to read the level of charge on the floating gate without causing any leakage of charge therefrom.
Various means are known in the art for introducing charge onto a floating gate and for removing the charge from the floating gate. Once the floating gate has been programmed at a particular charge level, it remains at that level essentially permanently, because the floating gate is surrounded by an insulating material which acts as a barrier to discharging of the floating gate. Charge is typically coupled to the floating gate using hot electron injection or electron tunneling. Charge is typically removed from the floating gate by exposure to radiation (UV light, x-rays), avalanched injection, or Fowler-Nordbeim electron tunneling. The use of electrons emitted from a cold conductor was first described in an article entitled Electron Emission in Intense Electric Fields by R. H. Fowler and Dr. L. Nordheim, Royal Soc. Proc., A, Vol. 119 (1928). Use of this phenomenon in electron tunneling through an oxide layer is described in an article entitled Fowler-Nordheim Tunneling into Thermally Grown SiO2 by M. Lenzlinger and E. H. Snow, Journal of Applied Physics, Vol. 40, No. 1 (January, 1969), both of which are incorporated herein by reference. Such analog floating gate circuits have been used, for instance, in digital nonvolatile memory devices and in analog nonvolatile circuits including voltage reference, Vcc sense, and power-on reset circuits.
FIG. 1A is a schematic diagram that illustrates one embodiment of an analog nonvolatile floating gate circuit implemented using two polysilicon layers formed on a substrate and two electron tunneling regions. FIG. 1A illustrates a cross-sectional view of an exemplary prior art programmable voltage reference circuit 70 formed on a substrate 71. Reference circuit 70 comprises a Program electrode formed from a first polysilicon layer (poly1), an Erase electrode formed from a second polysilicon layer (poly2), and an electrically isolated floating gate comprised of a poly1 layer and a poly2 layer connected together at a corner contact 76. Typically, polysilicon layers 1 and 2 are separated from each other by a thick oxide dielectric, with the floating gate fg being completely surrounded by dielectric. The floating gate fg is also the gate of an NMOS transistor shown at 73, with a drain D and a source S that are heavily doped n+ regions in substrate 70, which is P type. (The numeral zero is also referred to as xe2x80x9c0xe2x80x9d or xe2x80x9cØxe2x80x9d herein.) The portion of dielectric between the poly1 Program electrode and the floating gate fg, as shown at 74, is a program tunnel region (or xe2x80x9ctunnel devicexe2x80x9d) TP, and the portion of dielectric between the poly1 floating gate fg and the poly2 erase electrode, shown at 75, is an erase tunnel region TE. Both tunnel regions have a given capacitance. Since these tunnel regions 74,75 are typically formed in thick oxide dielectric, they are generally referred to as xe2x80x9cthick oxide tunneling devicesxe2x80x9d or xe2x80x9cenhanced emission tunneling devices.xe2x80x9d Such thick oxide tunneling devices enable the floating gate to retain accurate analog voltages in the +/xe2x88x924 volt range for many years. This relatively high analog voltage retention is made possible by the fact that the electric field in most of the thick dielectric in tunnel regions 74,75 remains very low, even when several volts are applied across the tunnel device. This low field and thick oxide provides a high barrier to charge loss until the field is high enough to cause Fowler-Nordheim tunneling to occur. Finally, reference circuit 70 includes a steering capacitor CC that is the capacitance between floating gate fg and a different n+ region formed in the substrate that is connected to a Cap electrode.
FIG. 1B is a schematic diagram that illustrates a second embodiment of a floating gate circuit 70 that is implemented using three polysilicon layers. The three polysilicon floating gate circuit 70xe2x80x2 is similar to the two polysilicon embodiment except that, for example Erase electrode is formed from a third polysilicon layer (poly 3). In addition, the floating gate fg is formed entirely from a poly2 layer. Thus, in this embodiment there is no need for a comer contact to be formed between the poly1 layer portion and the poly2 layer portion of floating gate fg, which is required for the two polysilicon layer cell shown in FIG. 1A.
Referring to FIG. 2, shown at 25 is an equivalent circuit diagram for the voltage reference circuit 70 of FIG. 1A and 70xe2x80x2 of FIG. 1B. For simplicity, each circuit element of FIG. 2 is identically labeled with its corresponding element in FIGS. 1A and 1B.
Setting reference circuit 70 to a specific voltage level is accomplished using two separate operations. Referring again to FIG. 1A, the floating gate fg is first programmed or xe2x80x9cresetxe2x80x9d to an off condition. The floating gate fg is then erased or xe2x80x9csetxe2x80x9d to a specific voltage level. Floating gate fg is reset by programming it to a net negative voltage, which turns off transistor TØ. This programming is done by holding the Program electrode low and ramping the n+ bottom plate of the relatively large steering capacitor CC to 15 to 20V via the Cap electrode. Steering capacitor CC couples the floating gate fg high, which causes electrons to tunnel through the thick oxide at 74 from the poly1 Program electrode to the floating gate fg. This results in a net negative charge on floating gate fg. When the bottom plate of steering capacitor CC is returned to ground, this couples floating gate fg negative, i.e., below ground, which turns off the NMOS transistor TØ.
To set reference circuit 70 to a specific voltage level, the n+ bottom plate of steering capacitor CC, the Cap electrode, is held at ground while the Erase electrode is ramped to a high voltage, i.e., 12 to 20V. Tunneling of electrons from floating gate fg to the poly2 Erase electrode through the thick oxide at 75 begins when the voltage across tunnel device TE reaches a certain voltage, which is typically approximately 11V. This tunneling of electrons from the fg through tunnel device TE increases the voltage of floating gate fg. The voltage on floating gate fg then xe2x80x9cfollowsxe2x80x9d the voltage ramp coupled to the poly2 Erase electrode, but at a voltage level offset by about 11V below the voltage on the Erase electrode. When the voltage on floating gate fg reaches the desired set level, the voltage ramp on poly2 Erase electrode is stopped and then pulled back down to ground. This leaves the voltage on floating gate fg set at approximately the desired voltage level.
As indicated above, reference circuit 70 meets the requirements for voltage reference applications where approximately 200 mV accuracy is sufficient. The accuracy of circuit 70 is limited for two reasons. First, the potential on floating gate fg shifts down about 100 mV to 200 mV after it is set due to the capacitance of erase tunnel device TE which couples floating gate fg down when the poly2 Erase electrode is pulled down from a high voltage to ØV. The amount of this change depends on the ratio of the capacitance of erase tunnel device TE to the rest of the capacitance of floating gate fg (mostly due to steering capacitor CC), as well as the magnitude of the change in voltage on the poly2 Erase electrode. This voltage xe2x80x9coffsetxe2x80x9d is well defined and predictable, but always occurs in such prior art voltage reference circuits because the capacitance of erase tunnel device TE cannot be zero. Second, the accuracy of circuit 70 is also limited because the potential of floating gate fg changes another 100 mV to 200 mV over time after it is set due to various factors, including detrapping of the tunnel devices and dielectric relaxation of all the floating gate fg capacitors.
An analog voltage reference storage device that uses a floating gate is described in U.S. Pat. No. 5,166,562 and teaches the uses of hot electron injection for injecting electrons onto the floating gate and electron tunneling for removing electrons from the floating gate. The floating gate is programmed by controlling the current of the hot electron injected electrons after an erase step has set the floating gate to an initial voltage. See also U.S. Pat. No. 4,953,928. Although this method of programming the charge on a floating gate is more accurate than earlier analog voltage reference circuits including a floating gate, the level of accuracy is still on the order of 50 mV to 200 mV.
Prior art floating gate storage devices have sometimes used dual conduction of Fowler-Nordheim tunnel devices, i.e., wherein both the program and erase tunnel elements in a floating gate device are caused to conduct simultaneously in order to provide the coupling of charge onto the floating gate. However, this method has only been used in digital circuits to program the floating gate to either a xe2x80x9c1xe2x80x9d condition or a xe2x80x9c0xe2x80x9d condition to provide memory storage. The precise charge on the floating gate in such applications is not of concern and so is not precisely controlled in such circuits. According to the prior art, such dual conduction digital programming of a floating gate is considered to be a less efficient and desirable way than generating electron conduction through a single tunnel element to control the level of charge on a floating gate. Known disadvantages of dual conduction digital programming of a floating gate include the fact that a larger total voltage is required to provide dual conduction and tunnel oxide trap-up is faster because more tunnel current is required.
An example of a prior art analog nonvolatile floating gate circuit that uses dual conduction of electrons for adding and removing electrons from a floating gate is disclosed in U.S. Patent No. 5,059,920, wherein the floating gate provides an adaptable offset voltage input for a CMOS amplifier. In this device, however, only one Fowler-Nordheim tunnel device is used. The electrons are injected onto the floating gate using hot electron injection, while Fowler-Nordheim electron tunneling is used to remove electrons from the floating gate, so as to accurately control the charge on the floating gate. This means of injecting electrons onto the floating gate is used because the charge transfer is a controlled function of the voltage on the floating gate. Another example of a prior art dual conduction floating gate circuit is disclosed in U.S. Pat. No. 5,986,927. A key problem with such prior art devices is that they do not compensate for common-mode voltage and current offsets, common-mode temperature effects, and mechanical and thermal stress effects in the integrated circuit.
Applications that require increased absolute voltage accuracy generally use a bandgap voltage reference. A bandgap voltage reference typically provides approximately 25 mV absolute accuracy over time and temperature, but can be configured to provide increased accuracy by laser trimming or E2 digital trimming at test. While a bandgap voltage reference provides greater accuracy and increased stability over the prior art voltage reference circuits discussed above, a bandgap voltage reference only provides a fixed voltage of about 1.2V. Therefore, additional circuitry, such as an amplifier with fixed gain, is needed to provide other reference voltage levels. Moreover, prior art bandgap voltage references typically draw a relatively significant current, i.e., greater than 10 xcexcA.
What is needed is an improved method for setting a floating gate in a floating gate circuit to a desired voltage such that a reference voltage can be provided that has an improved stability and accuracy over time and temperature as compared to prior art voltage references.
The present invention is a method for sensing the voltage on a floating gate while charge is being stored thereon in a floating gate circuit to enable programming of said floating gate to a precise voltage, said floating gate circuit including a differential circuit having a first transistor coupled to an inverting input of said differential circuit, wherein said floating gate is the gate of said first transistor, a second transistor coupled to a non-inverting input of said differential circuit, and a differential circuit output, said floating gate circuit further including a feedback circuit coupled from said output to said floating gate, said method comprising the steps of: a) causing said floating gate circuit to enter into a set mode, wherein a first predetermined voltage is coupled to the gate of said second transistor; b) causing the voltage on said floating gate to be sensed by said first transistor relative to said first voltage; c) causing an output voltage to be generated at said differential circuit output; d) causing the voltage on said floating gate to be modified as a function of said output voltage using said feedback circuit, including modifying the charge level on said floating gate under the control of a first tunnel device and a second tunnel device operating in dual conduction during said set mode, said first tunnel device formed between said floating gate and a first tunnel electrode and said second tunnel device formed between said floating gate and a second tunnel electrode; and e) repeating steps b) through d) until said floating gate circuit settles to a steady state condition such that the voltage on said floating gate is approximately equal to said first voltage.
According to an alternative embodiment, the present invention comprises a method for sensing the voltage on a floating gate while charge is being stored thereon in a floating gate circuit to enable programming of the floating gate to a precise voltage, said floating gate circuit including said floating gate, an erase tunnel device formed between said floating gate and an erase electrode, a program tunnel device formed between said floating gate and a program electrode, a differential circuit having a first transistor coupled to an inverting input of said differential circuit, wherein said floating gate is the gate of said first transistor, a second transistor coupled to a non-inverting input of said differential circuit, and a differential circuit output, said floating gate circuit further including a feedback circuit coupled from said output to said floating gate, said method comprising the steps of a) causing said floating gate circuit to enter into a set mode, wherein a first predetermined voltage is coupled to the gate of said second transistor; b) causing the voltage on said floating gale to be sensed by said first transistor relative to said first voltage; c) causing an output voltage to be generated at said differential circuit output; and d) causing said erase electrode to be biased by a second voltage that has a positive value that is generated as a function of said output voltage and causing said program electrode to be biased by a third voltage that has a negative value, such that said erase and program tunnel devices are operating in dual conduction for modifying the voltage on said floating gate as a function of said output voltage using said feedback circuit, and repeating steps b) through d) until said floating gate circuit settles to a steady state condition such that the voltage on said floating gate is approximately equal to said first voltage.
According to another embodiment, the present invention comprises an apparatus for sensing the voltage on a floating gate while charge is being stored thereon in a floating gate circuit to enable programming of the floating gate to a precise voltage. The apparatus comprises a first tunnel device connected to said floating gate to enable charge to be coupled to said floating gate; a second tunnel device connected to said floating gate to enable charge to be removed from said floating gate; a differential circuit including a first transistor configured as an inverting input of said differential circuit, wherein said floating gate is the gate of said first transistor, a second transistor configured as a non-inverting input of said differential circuit, and a differential circuit output; a feedback circuit coupled from said differential circuit output to said floating gate; and a circuit for coupling a predetermined voltage to the gate of said second transistor, said differential circuit operating in response to said predetermined voltage and the voltage on said floating gate to generate a voltage at said differential circuit output that is a function of the difference between said predetermined voltage and said floating gate voltage; said differential circuit and said feedback circuit operating to cause said first and second tunnel devices to be in dual conduction such that the charge level on said floating gate is modified as a function of said output voltage until said floating gate circuit settles to a steady state condition such that the voltage on said floating gate is approximately equal to said first voltage.
An object of the present invention is therefore to provide a method and apparatus for sensing the analog voltage on a floating gate to enable the precise setting of the floating gate to a desired voltage during a set mode.
A key advantage of the present invention is that a reference voltage that is programed using the present invention has an improved accuracy over prior art floating gate voltage references by more than a factor of 100.
Another key advantage of the present invention is that without the need for using laser trimming or E2 digital trimming, a reference that is programmed using the present invention provides for an improved accuracy over bandgap voltage references by a factor of 10 to 50 while drawing less power by a factor of more than 10.